The invention relates to the field of stretchable, flexible and conformable integrated devices, including electronics, photovoltaics, antennas, and other integrated devices. In particular the invention relates to substrate designs that exhibit regions with low strain when subjected to larger macroscopic strain.
Current, large-scale commercial technology for integrated electronic devices is primarily based on rigid (stiff) printed circuit boards. The rigid boards serve as the support for the electronic components and conductive interconnects (traces), and also limit the magnitudes of strain and stress that are transmitted to the components and traces. It is necessary to limit the strains and stresses transmitted to these components and traces to avoid mechanical failure, and to ensure continuous electronic function.
As electronic devices become increasingly pervasive in all aspects of life, researchers, inventors and industries are seeking alternative designs for providing stretchability, flexibility, and/or conformability to these devices (“flexible electronics”, “stretchable electronics”), in order to increase the design space and allow novel applications.
Several intermediate solutions exist, most notably commercial rigid-flex technologies, where rigid printed circuit boards are connected by flexible/ribbon interconnects. These solutions have only partially achieved the desired flexibility and stretchability, while introducing additional manufacturing/assembly challenges and costs.
Two general approaches have been taken in seeking a means to enable flexible electronics: (1) making conductors from a material that exhibits both conductivity and stretchability or flexibility, and (2) changing the base circuit board from a stiff material to a stretchable and/or flexible material. Due to fundamental limitations with the developments which have been made thus far, neither approach has yet to provide a large-scale commercial flexible/stretchable electronic technology.
With regards to the approach of using flexible conductive materials, several developments have been made using flexible conjugated polymers with conductive properties embedded/deposited/printed in/on a continuous stretchable substrate. Problems related to the attachment of commercial electronic components to the stretching substrate without transmission of forces that will tend to either fracture or detach the components have not been resolved in a manner compatible with current industrial electronic manufacturing processes. Furthermore, a fundamental limitation of this approach is that current polymeric conductive materials have excessive impedance, severely limiting the applications that can employ these materials. Metallic interconnects are currently the only solution compatible with a majority of electronic devices.
With regards to the approach of substituting the base circuit board with a flexible substrate, several developments have been made employing several methods to create “stretchable metallic interconnects.” Most commonly, unsupported “meandering” metallic interconnects, including serpentine and coiled configurations, are employed.
Stretchable devices using unsupported metal interconnects suffer from two primary problems. First, these approaches introduce a number of challenging intermediate fabrication steps, completely incompatible with conventional industrial electronic manufacturing processes. Second, these devices are prone to failure when deformed, as either the unsupported leads or the connection of the leads to the substrate carrying the electronic components tend to fail under the forces necessary to stretch the interconnects.
In another approach stretchable interconnects are created by embedding/depositing metallic traces in/on a continuous stretchable (elastomeric) substrate, laying out the circuitry in a wavy pattern in the plane of the substrate (sinusoidal or saw tooth type of pattern), which would unbend when stretched in one direction without inducing much strain on the circuitry. This approach only applies to the wire leads and does not address chips and other, functional elements.
Currently, several research groups have made a number of contributions in developing flexible electronics. The basic element of many of these designs is in a related approach, a thin conducting metallic film is used to create interconnects on a soft substrate. The conducting film has limited strain capability and is typically first put into a pre-compressed state by applying some form of pre-stretch which gives a final buckled or twisted structure for the metallic film. The film then accommodates macroscopic strain by “unbending” or “straightening out the undulations” when the multilayer film as a whole is strained in tension or other loading. Several groups have utilized pre-set out-of-plane buckling of conductive traces on a continuous stretchable substrate as a mechanism to enable stretchable circuits.
Stretchable interconnects technology where metallic traces are embedded in a wavy pattern along the surface of a continuous stretchable substrate present a number of shortcomings. If the traces are compliant (thin traces) the stretch of the traces is equal to the macroscopic stretch of the substrate, and limited deformations can be accommodated without compromising circuit integrity. If the traces are not compliant and limit the local level of deformation, interfacial stresses will arise that will tend to detach the traces from the stretching substrate. If the traces are wavy or pre-buckled out-of plane, the necessary manufacturing methods are not compatible with current industrial electronic manufacturing processes. Finally, problems related to the attachment of commercial electronic components to the stretching substrate without transmission of forces that will tend to either fracture or detach the components have not been resolved in a manner compatible with current industrial electronic manufacturing processes. The underlying limitation of this approach lies in the fact that the stretchable substrate is continuous, and therefore experiences local levels of stretch, around interconnects and electronic components, which are comparable to the imposed macroscopic stretch on the device.
In short, all of these approaches have failed in facilitating large-scale flexible/stretchable electronics production. Furthermore, different limiting factors make each approach incompatible with the current industrial manufacturing capabilities and unable to support circuit complexities comparable with the current generation of rigid, rigid-flex, and flex electronics.